Pharmaceutical companies currently screen millions of compounds looking for promising drug candidates such as agents that bind with high affinity to cellular receptors. Many binding assays (often fluorescence-based) being applied to this challenge suffer from drawbacks that limit their use. A technique examining changes in fluorescence polarization (FP) promises several advantages over other assays, including improved quantification of binding affinity between ligand and receptor, and increased simplicity and lower cost, due to its homogeneous (mix-and-read) format. Currently available FP-based instruments are not ideal, however, because of their low sensitivity and relatively slow operation. An approach, "Symmetric FP", developed at CRI, will lead to analytical instruments with greatly improved sensitivity and speed and should bring FP-based assays to the forefront of high-throughput screening methods. We propose to continue development of an FP high-density plate reader. Phase I and subsequent work have demonstrated 100-fold improvement in sensitivity using standard plastic microtitre plates and sample volumes as low as 1 mu1, and a more than 4-fold increase in plate-reading speed, compared to existing methods. The instrument will be particularly useful for developing drugs targeting clinically significant cellular receptors expressed at low levels in vivo. PROPOSED COMMERCIAL APPLICATION: Our plate reader would be used for drug discovery by pharmaceutical companies. The larger companies, of which there are at least 50, have between 20 and 50 drug screening programs ongoing at any given time. We expect that most of their existing ligand-screening instrumentation will be replaced by higher throughput, more cost-effective instruments, notably the proposed CRI FP platform. Conservative estimates suggest a potential market for 6-18 million dollars.